Beast Quest: Sting the Scorpion Man by Adam Blade

They watched in horror as the boy's legs fused together, forming a single limb that lengthened and tore through his clothes. It grew darker and harder until it turned into a bulbous, beaded tail. The tail swiped out powerfully at Tom and Elenna, knocking them both to the ground. Seth screamed again. His voice sounded deeper and rougher. Three legs forced their way out of both sides of his torso, tearing through his tunic.